Abstract

Local field-induced optical properties of Ag-coated CdS quantum dot structures are investigated. We experimentally observe a clear exciton peak due to the quantum confinement effect in uncoated CdS quantum dots, and surface plasmon resonance and red-shifted exciton peak in Ag-coated CdS composite quantum dot structures. We have calculated the Stark shift of the exciton peak as a function of the local field for different silver thicknesses and various sizes of quantum dots based on the effective-mass Hamiltonian using the numerical-matrix-diagonalization method. Our theoretical calculations strongly indicate that the exciton peak is red-shifted in the metal-semiconductor composite quantum dots due to a strong local field, i.e., the quantum confined Stark effect.

Figures (4)

Absorption spectra of pure CdS quantum dot of radius 1.3 nm and Ag-coated CdS quantum dot. Dashed and dotted lines are the measured absorption spectra of the pure and Ag-capped QDs, respectively. Solid line is the calculated absorption spectrum of the capped QD with silver thickness of 0.35 nm in the presence of an electric field 1×106V/cm.

Calculated Stark shifts of various electron-hole pair resonances as a function of the local field inside CdS QD (radius = 1.3 nm) coated with a 0.35 nm layer of silver. SS and PP: 1s(p)e and 1s(p)h exciton states with L=0 and L=1, respectively, SD and DS: 1s(d)e and 1d(s)h exciton states with L=2.

Calculated absorption spectra of CdS/Ag quantum dots of radius 1.3 nm for various silver thicknesses of 0.35 nm, 0.40 nm and 0.45 nm. The solid line is the same absorption spectrum as in Figure 1 for a Ag thickness of 0.35 nm.